Keysight Technologies U8480 Series Service Manual page 35

Theory of Operation 3
Figure 3-1 illustrates the block diagram of the AC-coupled U8480 Series. The
U8480 Series input consists of the bulkhead assembly which has a built-in GaAs
thermocouple IC for optimum VSWR and isolation performance. The bulkhead
assembly has embedded a thermistor for close temperature monitoring and a DC
block for the standard model.
The heat generated by the RF/μW signal in the load termination of the
thermocouple is converted to a differential signal by the thermopile which is then
fed into a differential chopper, gain stages, and to a 4-channel 24-bit
analog-to-digital converter (ADC) in the later processing stage.
The U8480 Series control and processing is provided by a DSP embedded
processor which is supported by a 64 MB SDRAM. The available processing power
enables the implementation of correction algorithm such as the linearity
correction, calibration factor, and temperature compensation algorithm. The
correction factors, firmware, and default settings for the U8480 Series are stored
in the 3 MB non-volatile memory.
The U8480 Series is equipped with an internal calibration function which enables
the U8480 Series to perform self-calibration without the need of the 1 mW 50 MHz
reference calibrator. During the internal calibration process, the switch for path
selection directs the analog circuitry to the on-board reference circuit. The
reference circuitry provides a very accurate source that has negligible long term
drift during the U8480 Series calibration cycle which is used to correct for analog
circuitry drift and aging. The analog circuitry correction is combined with the
linearity correction algorithm to achieve the internal calibration process. Internal
calibration is allowable if there is RF input to the U8480 Series; it will not impact
the U8480 Series performance.
The trigger input port which is based on TTL enables the U8480 Series to
synchronize with events. Communications and remote control of the U8480 Series
is via the Universal Serial Bus (USB) connectivity which is USB-TMC compliant.
Keysight U8480 Series Service Guide 35